1. Field of the Invention
The invention relates to low light level television (LLL/TV) cameras particularly with respect to apparatus utilizing a focal plane array optically coupled to an image intensifier.
2. Description of the Prior Art
LLL/TV sensors of present day design utilize an image intensifier with the output image thereof optically coupled to a solid state focal plane array sensor. Preferably, a second-generation image intensifier is utilized having a gateable input photocathode with an input image plane, a microchannel plate and an output phosphor with an output image plane. An input image is focussed on the input image plane, the photons of the input image resulting in electrons emitted from the photocathode. The microchannel plate accelerates the electrons to impinge upon the phosphor output resulting in an intensified image on the output image plane. Thus, the image intensifier provides a luminous gain to the input image. The output image is formed on the CRT-type phosphor which is typically green. The focal plane array generally comprises a matrix of charge coupled devices (CCD).
Two methods are utilized in the prior art to relay the output image from the image intensifier phosphor image plane to the focal plane array. A conventional lens system or a section of coherent fiber optic image conduit material is utilized. The relay lens apparatus, utilizing conventional lenses, results in an assembly which is unacceptably large and heavy. Additionally, such assemblies tend to have alignment and focus problems and are not mechanically rugged.
With the fiber optic approach, the output face of the fiber optic bundle is fastened to the focal plane array and the input face thereof is contacted to the image intensifier output to relay the intensified image. Although the fiber optic method operates adequately, the fiber optic bundle requires intimate contact with the focal plane array surface. Any space larger than several thousdandths of an inch results in defocussing. A commercially procurable focal plane array is mounted in a housing having a transparent input window such as glass. The prior method traditionally utilized to combine the image intensifier and the focal plane array into an LLL/TV sensor required that the focal plane array window be removed and the fiber optic image conduit be cemented, or otherwise attached, directly to the semiconductor surface. The focal plane arrays are often damaged in the fiber optic attachment process resulting in an unacceptably low yield.
In addition to the above, a new state of the art class of improved focal plane arrays are currently available which render fiber optic coupling impossible. The new focal plane arrays have a glass filter element cemented directly on the focal plane array semiconductor surface, so that intimate contact therewith by the fiber optic conduit is impossible. The glass filters are typically fifty-thousandths of an inch thick and would result in defocussing were the fiber optic bundle attached thereto.